In popular music, audio sources are frequently reproduced in reverse as part of a passage in a musical piece. This effect is typically achieved by recording an audio source onto magnetic tape and then playing the tape backwards on a special tape recorder. The sound of an audio source reproduced in reverse, such as a human voice, produces an interesting and often pleasing effect within a musical piece.
More recently the effect has been achieved, particularly in rap-style music, by playing a phonograph record backwards. This is accomplished by playing a phonograph record in its normal clockwise direction and, when the effect is desired, grasping the record and pushing it in a counter-clockwise rotation momentarily so that the phonograph needle reproduces the recorded signal in reverse. This technique is commonly known as "scratching", presumably due to the type of sound produced, and is frequently used in rap-style music.
The effect of playing a sound sample in reverse cannot be accomplished in real time with the existence of the original sound source because a sound sample must first be recorded in its entirety before the sample can be reproduced in reverse. A sound cannot be reproduced, in forward or reverse, before it exists.
Further, since the advent of compact disc technology, phonograph records are becoming increasing less common. Accordingly, the availability of LP records for performing "scratching" is becoming a problem.
U.S. patent application Ser. No. 07,889,648 now U.S. Pat. No. 5,245,663, discloses a back-masking effect generator which simulates the sound of playing an audio input signal in reverse almost in real time. The device disclosed in that patent comprised two RAMs, each having associated therewith a counter, an input register and an output register. The device further comprises an analog-to-digital converter for receiving an input audio signal and providing a sampled digital output stream to the RAMs, and a digital-to-analog converter at the outputs of the RAMs for encoding analog signals from the RAM's digital contents. All of the components are controlled by a finite state machine.
In operation, at startup, a first one of the RAMs receives the digitally converted input audio data from the analog-to-digital converter and stores it incrementally in memory. When the first RAM is full, the apparatus is switched to a first one of two continuous operating states. In the first continuous operating state, the first RAM's memory is read out to the digital-to-analog converter in the reverse order from that in which it was written while simultaneously, the second RAM is incrementally filled with the output data from the analog-to-digital converter.
The two RAMs operate at the same speed so that the first RAM will be completely empty at the same time that the second RAM becomes completely filled. At that point, the device enters the second continuous operating state in which the operation is reversed so that the second RAM is emptied into the digital-to-analog converter in the reverse order from which it was filled, while the first RAM receives and incrementally stores data from the analog-to-digital converter. The finite state machine then continuously switches between the first and second continuous operation states until the device is otherwise disabled.
The device disclosed in that application, however, is inefficient in that it requires two RAMs, two counters and two sets of input and output registers.
Therefore, it is an object of the present invention to provide a back-masking effect generator which simulates the sound of playing an input audio source in reverse almost in real time utilizing only a single memory device and related hardware.
It is another object of the present invention to provide a back-masking effect generator in which the length of each packet of reverse signal is adjustable.